Processing, release and metabolism of cholecystokinin in SK-N-MCIXC cells

Cleavage-site mutagenesis alters post-translation processing of Pro-CCK in AtT-20 cells

Cholecystokinin (CCK) is expressed in the central and peripheral nervous systems and functions as a neurotransmitter and neuroendocrine hormone. The in vivo forms of CCK include CCK-83, -58, -39, -33, -22, -12, and -8. Tissues in the periphery produce the larger forms of CCK, such as CCK-58, whereas the brain primarily produces CCK-8. The different biologically active forms of CCK observed in vivo may result from cell-specific differences in endoproteolytic cleavage during post-translational processing. Evidence suggests that cleavages of pro-CCK occur in a specific sequential order. To further delineate the progression of cleavages during pro-CCK maturation, mutagenesis was used to disrupt putative mono- and dibasic cleavage sites. AtT-20 cells transfected with wild-type rat prepro-CCK secret CCK-22 and -8. Mutagenesis of the cleavage sites of pro-CCK had profound effects on the products that were produced. Substitution of basic cleavage sites with nonbasic amino acids inhibits cleavage and leads to the secretion of pathway intermediates such as CCK-83, -33, and -12. These results suggest that CCK-58 is cleaved to both CCK-33 and -22. Furthermore, CCK-8 and -12 are likely derived from cleavage of CCK-33 but not CCK-22. Alanine substitution at the same site completely blocked production of amidated products, whereas serine substitution did not. The cleavages observed at nonbasic residues in this study may represent the activity of enzymes other than PC1 and carboxypeptidase E, such as the enzyme SKI-1. A model for the progression of pro-CCK processing in AtT-20 cells is proposed. The findings in this study further supports the hypothesis that pro-CCK undergoes parallel pathways of proteolytic cleavages.

Identification of overlapping AP-2/NF-kappa B-responsive elements on the rat cholecystokinin gene promoter

In this study we evaluate both proximal and more distal transcriptional regulation of the 5' flanking region of the rat cholecystokinin gene in transfected GH3 (rat pituitary tumor) cells. Transcriptional activity was measured on the intact (-400 to +73) 5' flanking region of cholecystokinin (CCK), as well as with DNA constructs, which were deleted in both the conventional 5' to 3', as well as an unconventional 3' to 5' direction. Our in vivo studies indicate complex phorbol ester and forskolin interactions in the 10-base pair region between -130 and -140. We conclude, there are at least two transcriptional factors involved in regulation of the rat CCK transcription in this region. In vitro studies utilizing heterologous nuclear (HeLa) extract, as well as purified transcription factors AP-2 and NF-kappa B, identify overlapped AP-2- and NF-kappa B-responsive elements within the 17-base pair sequence between -149 and -134 of the distal 5' flanking region. In this region complex transcriptional regulation occurs, which indicates inhibition of AP-2 CCK promoter complexing by NF-kappa B. Six-point mutations introduced into this sequence prevent AP-2 and NF-kappa B binding to CCK promoter, as well as its transcriptional activation by phorbol ester and forskolin in GH3 cells.

Neuroendocrine protein 7B2 is essential for proteolytic conversion and activation of proprotein convertase 2 in vivo

The 7B2 protein is widely distributed in neural and endocrine tissues. Its biological function was found to be related to the processing enzyme proprotein convertase 2 (PC2), a mammalian subtilisin/kexin-like endoproteinase that cleaves at specific single or multiple basic amino-acid residues. In order to examine the proposed function of 7B2 on PC2 in in vivo models, we first compared the distribution of 7B2 and PC2 mRNAs in the rat brain. Expression of 7B2 mRNA was found to be pan-neuronal, but additionally, we observed 7B2 mRNA in ependymal cells and in the subcommissural organ. Although the expression of PC2 mRNA was exclusively neuronal, it was more restricted, sparing some regions expressing high levels of 7B2. This finding suggests that 7B2 has an additional function in non-PC2-expressing cells. No evidence of PC2-positive/7B2-negative cells could be obtained in the adult rat brain. However, in the developing rat brain (E17), such regions were easily observed, showing higher levels of pro-PC2 (75 kD). Similarly, in the animal model of insulin-induced hypoglycemic shock, where adrenomedullary 7B2 expression is decreased, the ratio of pro-PC2 to mature PC2 (75 kD:68 kD) was observed to be increased. Finally, the human neuroepithelioma SK-N-MCIXC expresses PC2 but not 7B2. Accordingly, only inactive pro-PC2 forms were observed: 75-kD intracellular and 71-kD extracellular. After stable transfection of SK-N-MCIXC cells with 27-kD pro-7B2, mature and active (68-kD) PC2 was secreted into the medium. Our data demonstrate a critical role of 7B2 in the proteolytic conversion and activation of PC2 in vivo.

Differential modulation of prohormone convertase mRNA by second messenger activators in two cholecystokinin-producing cell lines

Regulation of prohormone convertase expression was studied in two neuropeptide-producing cell lines, the human neuroepithelioma cell line SK-N-MCIXC and the rat medullary thyroid carcinoma cell line WE 4/2. The cells were treated with the phosphodiesterase inhibitor isobutyl-methylxanthine and the tumor-promoting phorbol ester, phorbol-12-myristate-13 acetate, activators of the cyclic AMP (cAMP) and protein kinase C (PKC) second messenger pathways, respectively. mRNA levels of prohormone convertase 1 (PC1), prohormone convertase 2 (PC2), and furin were determined after 3- and 6-h treatments, using Northern analysis. Activation of both cAMP and PKC pathways increased PC1, but not PC2 or furin mRNA levels in SK-N-MCIXC cells. Activation of the cAMP pathway increased PC1, PC2, and furin mRNA levels in WE 4/2 cells, whereas activation of the PKC pathway did not change prohormone convertase mRNA levels in this cell line. These results indicate that prohormone convertases may be differentially regulated by cAMP and PKC mechanisms and regulation may be tissue specific.

Modulation of CCK mRNA in cell lines in response to isoproterenol and retinoic acid

Regulation of cholecystokinin (CCK) expression was studied in the human neuroepithelioma cell line SK-N-MCIXC and the rat medullary thyroid carcinoma cell line WE 4/2. The cells were treated with the beta-adrenergic agonist isoproterenol and retinoic acid, a natural derivative of vitamin A, which plays a role in cell growth and proliferation. Levels of CCK mRNA were determined after 6, 12 and 24 h drug treatments, with Northern blot analysis using human CCK riboprobes. In WE 4/2 cells no differences were observed in CCK mRNA levels, between control and isoproterenol treated cells, after 6, 12 or 24 h treatments. In SK-N-MCIXC cells isoproterenol increased CCK mRNA levels at all time points examined, the beta-adrenergic antagonist propranolol blocked this effect. SK-N-MCIXC cells were also treated with actinomycin D or cycloheximide in combination with isoproterenol. Actinomycin D decreased CCK mRNA levels. Cycloheximide increased CCK mRNA levels when compared to isoproterenol acting alone. Retinoic acid did not affect CCK mRNA levels in WE 4/2 cells. In SK-N-MCIXC cells, retinoic acid consistently decreased CCK mRNA level. CCK mRNA levels in SK-N-MCIXC cells treated with retinoic acid combined with either isoproterenol or phorbol-12-myristate-13 acetate, were not significantly different from cells treated with retinoic acid alone.

Regulation of CCK mRNA in the human neuroepithelioma cell line SK-N-MCIXC in response to second messenger activators

Regulation of cholecystokinin (CCK) expression was studied in the human neuroepithelioma cell line SK-N-MCIXC. The cells were treated with the phosphodiesterase inhibitor isobutyl-methylxanthine and the tumor promoting phorbol ester, phorbol-12-myristate 13-acetate; activators of the cyclic AMP (cAMP) and protein kinase C (PKC) second messenger pathways, respectively. Levels of CCK mRNA were determined after 6, 12 and 24 hour drug treatments, with Northern blot analysis using human CCK cDNA hybridization probes. Activation of both cAMP and PKC second messenger pathways increased CCK mRNA levels in SK-N-MCIXC cells. These results indicate that the levels of CCK mRNA in SK-N-MCIXC cells are regulated by cAMP and PKC dependent mechanisms.